1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
A conventional wireless communication system includes one or more base stations (or Node-Bs) that may provide wireless connectivity to one or more mobile units. The mobile units may include cellular telephones, personal data assistants, smart phones, text messaging devices, pagers, network interface cards, notebook computers, desktop computers, and the like. The mobile units and the base stations communicate by exchanging information over an air interface (or wireless communication link) that typically includes a number of channels, such as traffic channels, signaling channels, paging channels, and the like. However, the resources available to provide wireless connectivity are limited, which may limit the total load that may be supported by the base station and/or the wireless communication system.
Operation of the base stations may be coordinated by a controller such as a radio network controller (RNC), which is typically used in wireless communication systems that operate according to Universal Mobile Telecommunication System (UMTS) standards and/or protocols. One of the essential functions of the radio network controller is load control. The basic task of load control is to prevent overload by means of admission control and to overcome overload situations by means of congestion control. The main uplink resource to be managed by load control algorithms is uplink interference, which may be represented by measurements of a Received Total Wideband Power (RTWP), which is typically measured in dBm. For example, in a wireless communication system that operates according to the UMTS Third Generation Partnership Project (3GPP) standard, measurements of the RTWP are filtered and pre-processed in a receiving base station and then reported to the RNC via a Node-B Application Part (NBAP) Measurement Reporting message.
FIG. 1 shows an example of conventional uplink load control handling in a radio network controller. First, the measurement is converted into a system load using the following relation:
                    load        =                              1            -                          1              NR                                =                      1            -                                                            RTWP                  0                                RTWP                            .                                                          (        1        )            In equation 1, the noise rise (NR) is computed using the difference between the measured RTWP and an estimated background noise RTWP0 (also in dBm). Then, depending on the result, the load control algorithms may perform one or more actions. In the low load region, i.e. when load<thr_DBC, the admission control algorithm in the radio network controller may admit all new traffic requests. In the medium load region, i.e. when thr_DBC<=load<thr_CAC, the admission control algorithm may admit only traffic requests with lower resource consumption, e.g., traffic requests associated with transmissions that use a lower data rate. In the high load region, i.e. when thr_CAC<=load, the admission control algorithm in the radio network controller may block all incoming call requests. In the very high load region, i.e. when thr_ConC<=load, a congestion control algorithm in the radio network controller may reduce the load by reducing the data rate of one or several users. In the extreme case, connections of one or several users could be released by this procedure.
FIG. 2 shows a conventional uplink load that may be used for load control of legacy dedicated channel (DCH) users. The vertical axis indicates increasing uplink load, which may be obtained as indicated by equation 1. The value of the variable total_load indicates the contributions to the load from legacy DCH users plus some smaller portion from users on random access channels (RACH). Accordingly, the value of total_load will vary depending on the uplink loading of the system and hence provides a reasonable measure for performing the appropriate load control decisions. The radio network controller may therefore use the value of the variable total_load as input to the load control and/or congestion control algorithms, which may attempt to maintain the value of the variable total_load near a predetermined threshold load indicated by the value of the variable Thr_load.
In some wireless communication systems, additional channels may be used to support data transfer over the uplink. For example, the UMTS 3GPP standards define an enhanced dedicated channel (E-DCH), which is an enhancement of the uplink dedicated channel (DCH) with focus on more efficient support of packet switched (PS) data in the uplink. The E-DCH in the uplink corresponds to the high speed downlink shared channel (HS-DSCH) transport channel in the downlink, which also increases the efficiency of the PS data transmission as compared to the legacy dedicated channels. Transmissions over the enhanced dedicated channel are scheduled by an E-DCH scheduler that resides in each Node-B that supports the enhanced dedicated channel.
The E-DCH scheduler in each base station is responsible for controlling loads associated with E-DCH users in the serving cell and E-DCH users that are in nearby cells. For example, the E-DCH scheduler may attempt to keep the sum of all the loads (including the background noise) within a target value that is typically signaled from the RNC to the base station. The E-DCH scheduler may also attempt to keep the ratio of the load from E-DCH users of other cells and the total E-DCH load at a certain level, which is also signaled from RNC. The E-DCH scheduler may also apply the constraint that only resources that have been leftover from the other users can be taken for scheduled E-DCH users.
However, some portions of the load (such as portions of the load associated with legacy dedicated channels) are not under the control of the E-DCH scheduler. The 3GPP UMTS standards have not specified new measurements that may be used by the radio network controller to determine, and thereby control, the portions of the load that are not under the control of the E-DCH scheduler but are controlled by RNC. In particular, conventional radio network controllers are unable to determine the portion of the load that should be under control of the radio network controller. Consequently, load control in conventional radio network controllers uses RTWP measurements to determine the UL load.
Performing uplink load control at the radio network controller using RTWP measurements has a number of drawbacks. Given a sufficient traffic volume on the E-DCH, the E-DCH scheduler will try to allocate all remaining resources that have been left over from the uplink users not under its control. Consequently, the value of the total load determined using the RTWP measurements would remain near the target value of the load and the radio network controller would always determine that the resources are properly allocated, regardless of the relative values of the loads associated with E-CH users and other users.
One potential solution to this problem is to set the threshold value thr_CAC for load control to a value that is approximately equal to the target load value. However, when the nc_load from the UL users that are not controlled by the E-DCH scheduler increases, the load control will not perform any action before nc_load reaches the target load and hence the difference between the target load and nc_load becomes smaller. Because the E-DCH scheduler can only control this difference between the target load and nc_load, which has been leftover from the other users, the available resources, which can be used by E-DCH, may decrease down to zero. Consequently, in that case the E-DCH scheduler may reduce the resources allocated to all users down to zero before any load control action is taken. Furthermore, due to the reporting of nearly constant RTWP (and therefore a nearly constant UL load), the radio network controller may be unable to perform data rate specific load control actions that were used in legacy DCH load control. In consequence, it is believed that high data rates on DCH, e.g. data rates of about 384 k or larger, cannot be supported with the current E-DCH load control when E-DCH is used in the same cell.